The invention is in the optical signal processing field. More particularly, the purpose of the invention is regeneration of an optical information transmission signal.
FIG. 8 very diagrammatically shows a regenerator 20 in which a signal is input from a transmission line L. The function of the regenerator is to regenerate the signal while amplifying it, reshaping it, and putting it back into phase. This function is known under the name of the 3R reconstitution standing for xe2x80x9creamplification, reshaping, retimingxe2x80x9d. Regeneration may comprise solely some regeneration functions, for example reshaping or reamplification. The regenerator 20 comprises a detector 21 that converts the received optical signal into an electrical signal carrying the optical signal modulation, a clock reconstitution means 22 that reconstitutes the clock signal from the signal, a means 23 of reconstituting the shape of the signal modulation received by line L, and finally a modulator 24 receiving firstly a carrier wave from a local oscillator 100 shown in dotted lines and the reshaped electrical signal from the means 23 of reconstituting the modulated shape of the signal received by line L to give a reconstituted retransmitted signal at the output.
In this device 20, it is noted that the carrier wave of the retransmitted signal is reconstituted locally, and therefore can differ from the initial carrier wave by modifications to characteristics such as the wave length that might be slightly different even if the two wave lengths have the same nominal value, polarization and phase.
Another example embodiment is described in the U.S. Pat. No. 5,781,326 belonging to the applicant issued on Jul. 14, 1998 (EP 813 097 A1). The purpose of the invention described in this patent is to propose a fully optical regeneration device, in other words without any optical-electrical or electrical-optical conversion.
The device for shaping a binary input signal in the shape of a first optical wave modulated between low and high power amplitudes, comprises:
a first stage to supply a modulating optical signal in the shape of a second optical wave with a determined wave length and modulated between first and second power amplitudes, as a function of the said input signal, the highest of the said first and second levels being stabilized to be only slightly dependent on fluctuations of the said low and high levels of the input signal, and
a second stage comprising an interferometric structure coupled with the said first stage to receive the said modulating signal and designed to supply an output signal resulting from constructive or destructive interference of the first and second auxiliary waves when the power of the modulating signal is equal to the said first or second levels respectively.
Each of the first and second stages of this shaping device can be used separately to produce an output signal regenerated from the input signal. The first and second stages can naturally be cascaded as described in this patent to give an improved regenerated signal, in other words capable of starting from a poor quality modulated input signal and producing an output signal in which the high levels are stabilized with a constant optical power, and in which the low levels have a practically zero power while presenting a very high signal-to-noise ratio.
For a good understanding of the benefits of the invention in the device described in this patent, each stage of the device described in this patent is shown in FIG. 1 in this application.
The first stage is shown in part A and the second stage is shown in part B.
The first stage 61 receives an input optical signal E in the form of a continuous optical wave xcexe modulated in a binary manner.
The first stage 61 comprises an optical amplifier with a fiber 63 that receives the input signal E and supplies an amplified input signal AE to a clipping device 64. The clipping device 64 comprises a semiconductor optical amplifier OA, one face of which is coupled to a laser oscillator 66 supplying an intermediate carrier wave L. The other face of amplifier OA is connected to a circulator 5 comprising a first port into which the amplified input signal AE from the fiber amplifier 63 is input, and a second port from which it is injected into the amplifier OA. A third port in the circulator 65 supplies the signal B through a rejecter filter F adjusted to the wave length xcexe of the input signal E. The wave length xcexb of the intermediate wave L is chosen to be different from the wave length of signal E.
Signal B may constitute the regenerated signal or a modulating signal for the second stage represented in part B that will now be described.
The second stage 62 of the device in FIG. 1B comprises an interferometric structure 67 composed of two guide branches provided with semiconductor optical amplifiers OA1 and OA2 respectively. A first coupler K1 couples one end of each of these branches to a laser source 68 supplying an output carrier wave M with wave length xcexs. A second coupler K2 is arranged so that the signal to be regenerated can be input into the first amplifier OA1. A third coupler K3 connected to the coupler K2 and to the second amplifier OA2 is arranged to supply an output signal S resulting from coupling of auxiliary waves AM1 and AM2 supplied by amplifiers OA1 and OA2 respectively. The waves AM1 and AM2 correspond to waves M1 and M2 output from coupler K1 and amplified by amplifiers OA1 and OA2 respectively.
Currents I1 and I2 are injected into amplifiers OA1 and OA2 respectively. According to a first option, these currents are adjusted such that the output signal S is the result of constructive interference between waves AM1 and AM2 when the power of the input signal is low, and is otherwise the result of destructive interference.
According to another option, the current I2 may be adjusted to a value I21 greater than I20 to obtain destructive interference when the power of the input signal is low, and constructive interference when it is high.
Regardless of the specific case, as shown in FIG. 1A or 1B or the case described in the patent mentioned above in which the signal B forms the input signal to the device shown in part B, the output wave length, as in the device described with reference to FIG. 8, is different from the input wave xcexe. Similarly, the polarization and phase of the output wave vary with respect to the input wave.
U.S. Pat. No. 5,781,327 delivered to TRW inc. describes an electro-optical modulation system operating in a closed loop. The carrier optical wave is generated by an optical amplifier medium 12 shown in FIGS. 1 and 5, and 32 in FIG. 3. This amplifying medium is pumped by a continuous wave, which is the continuous wave at the output from the amplifier medium connected to the input of this medium through the loop.
This cycle of the loop comprises means 10 of modulating the continuous wave output from the amplifier optical medium. The loop has a loop output for the modulated optical wave. The loop cycle comprises filter or demodulation means to reduce the width of the pumping wave band. This thus improves the optical efficiency of the overall device (column 2, row 43). In particular, it avoids the need to reinput unwanted resonance into the modulator that would disturb modulation (end of claim 1 in this patent).
The amplifying optical medium in this patent thus behaves like a local oscillator comprising a reaction loop, in which part of the output is returned to the input. This patent is mentioned because it reveals some filter means that are used in the invention that will be described later.
In regeneration devices, variations in the phase, wave length or polarization due to local generation of the beat wave, can introduce some disadvantages particularly in a WDM (Wavelength Division Multiplex) transmission system. Similarly, devices for coherent processing of the optical signal in which the polarization of the received signal is not known are shown in FIG. 9. In the device 40 shown in this figure, a polarized carrier wave TE and a polarized carrier wave TM are injected into the signal arriving on a line L using local oscillators 28, 29 and mixers 26, 27 respectively, to be sure that it beats with a wave with the same nominal wave length as a signal carrier wave. Thus, the signal carried by the carrier wave is detected. The electrical signals output from the mixing detectors 26 and 27 are then added in a known manner in an adder 31.
The invention is intended to make improvements to a transmission line, and in particular it is intended to keep the wave length of the carrier wave constant throughout the length of a transmission line. According to the invention, this is done by substituting a continuous wave extracted from the modulated carrier wave at an input to a signal regenerator transmitted through the said transmission line, to replace the continuous wave generated locally by a local oscillator. If each of the regenerators is conform with the invention, then the wave length of the initial carrier wave is conserved with its phase and its polarization throughout the length of the transmission line. When it is said that the phase and the polarization are conserved, the meaning is that the phase shift or the polarization shift with respect to the transmitted wave is constant.
This conservation has advantages in the case of a WDM transmission, but also advantages for optical noise on the line.
Thus, the invention relates to an optical telecommunication regenerator, the regenerator having:
an optical input to receive a continuous optical wave carrying a modulation signal, from another regenerator or a remote transmission source through an optical transmission line (L),
means of generating a pure optical wave,
a modulator receiving a modulation representative of the modulation received at the input, an output from the modulator being coupled to an output from the regenerator,
this regenerator being characterized in that the means of generating the pure optical wave are composed of extraction means into which the modulated optical wave from the said other regenerator or the said remote transmission source is input, these means processing this wave to reconstitute the carrier wave.
In their simplest form, the means of extraction of the carrier wave comprise low pass filter means into which the modulated signal is input, and which output the continuous optical wave generated by the generator. The terms high pass and low pass filters refer to low or high frequencies with respect to the base band of the carried signal. The low pass filter could be made using a Fabry-Perot cavity in which one of the nominal resonant frequencies is equal to the nominal frequency of the carrier wave received by the regenerator, this cavity receiving the modulated signal as input and producing a continuous optical wave as output. Depending on the nature of the original signal and the quality to be obtained for the regenerated carrier wave, it would also be possible to have a high pass filter into which the optical wave from the low pass filter is input, in this case this optical wave is a first intermediate continuous wave instead of the output continuous wave. The high pass filter eliminates any low frequency components that are still present in this intermediate continuous wave. In this case the output signal from the high pass filter forms the continuous optical wave generated by the means of generating a pure optical wave. It will be seen later that the high pass filter may be constituted by a semiconductor optical amplifier.